The old saying “There’s no replacement for displacement” is truly taken to heart by drag racers. For many years, top class racers pushed their factory engine platforms by increasing the bore and stroke to its limit, and then adding the best bolt-ons, nitrous or forced induction, and the best heads on the market. With power goals now reaching well beyond 3,000 horsepower in multiple classes, engine failures are not a matter of if, but when.
If you want ultimate horsepower with some type of reliability, you’re going to need to upgrade your engine platform to something customized to your needs. We sat down with Scott Kieffer of CFE Racing to get the inside scoop on using altered bore spacing engine platforms, in addition to the effects of using stronger materials on block rigidity, elasticity, and repairability.
An altered bore space block changes the centerline to centerline distance between two neighboring cylinders, as a means of running a larger bore to increase displacement more reliably.
Utilizing an altered bore spacing will increase displacement by utilizing a bigger bore and increased stroke, and allow for more power, higher boost, and overall greater cylinder pressure, all while increasing the structural integrity of the block itself.
“An altered bore spacing engine platform is a modified version of the O.E.M. architecture of a particular engine,” stated Kieffer. “Most notable in CFE’s altered bore space lineup are the Chevy small-block and big-block, which have a factory bore spacing of 4.400-inches and 4.840, respectively. We took those factory configurations and created competition blocks and cylinder heads in 4.500 and 4.600-inch bore space configurations for small-blocks, and 5.000, 5.200 and 5.300-inch versions for the big-block.”
All of CFE’s blocks are designed using 6061-T6 billet aluminum, which is much more durable and easier to repair than the factory cast design.
These bigger displacement engines with power adders have been considered somewhat perishable in respect to the amount of power they make. They do burn pistons and scuff cylinder walls rather easily; and being a detonation sensitive engine that is more prone to damage, they need to be easier to maintain and repair as well.
“There are two primary advantages to running a billet block over the standard cast design, the first being durability,” explains Kieffer. “The billet aluminum material is actually very elastic, it has an elongation (stretch before ultimate failure) of 25 to 30 percent. This allows the block and heads to flex to an extent, where as a cast piece is very rigid, and when exposed to the same environment as a billet block will fracture or crack much more easily.”
Left: A 6061-T6 billet aluminum engine block with a window punched in the side of it by a failed connecting rod. Right: A patch that will be welded in place to return the block to service. Photos courtesy of Miller Welders.
“The second advantage is the billet materials repairability,” Kieffer continues. “In a catastrophic engine failure, component failures such as a broken rod or severe detonation will break pieces of the block and heads apart.” Using a billet engine platform provides much better results when executing a repair in this situation, unlike a cast piece which tends to have dirt contamination and material imperfections that make welding unreliable or even impossible in some cases.
Left: The patch being welded into place. Right: The raw finished patch. After running this block through a mill, it will look brand new like the patch was never there. Photos courtesy of Miller Welders.
“6061-T6 aluminum alloy has good mechanical properties and exhibits good weldability, with the added benefit of also being one of the most common aluminum alloys used, making it very easy to acquire in various dimensions.”
Because of its repairability, using a billet altered bore space block is the more economical way to approach a power adder build or anything that is conducive to engine failure because of the power level the racing class is capable of.
Strength In Aluminum
There are countless ways an altered bore space block can be designed, even down to the thickness of the base metal between the cylinders – it all depends on what the goals are for your build.
CFE’s LS Fusion billet aluminum block with a 4.500″ altered bore spacing. This block is an LS/SBC hybrid.
“A billet aluminum block is the way to go for any racing application using power adders. Additionally, by using a bore spacing greater than O.E.M. we’re able to achieve several different advantages,”Kieffer elaborates. “An altered bore spacing block can provide a more rigid cylinder wall, but it really depends on the objective for the engine.”
“If you wanted to run in Mountain Motor Pro Stock, we would focus on maintaining a more round and straight bore, improving ring seal to adequately fill the cylinder,” says Kieffer. “For example, keeping the parent metal between the bores to a minimum and increasing the thickness of the sleeve, would theoretically create a more rigid cylinder wall to improve ring seal.”
The 1990s through the early 2000s was a tough period for a lot of Pro Mod class racers. Aftermarket engine platforms just couldn’t keep up with the improvements in the efficiency and size of power adders, causing racers to go through engine blocks at a much faster rate than ever before.
“With more efficient turbo, supercharger, and nitrous systems, better flowing cylinder heads, improved intake manifold designs, increased RPM, and more refined tuning capabilities flooding the industry at the time; the weak link in the engine block became the thin parent metal between the cylinder walls, which would frequently fail,” says Kieffer.
“It was just a very common problem that racers had to deal with at the time,” Kieffer recalls. “Using an altered bore spacing on a nitrous build gives us the ability to manipulate the parent metal between the sleeves to increase the thickness of the block itself. Spreading the bores apart while maintaining the same cylinder diameter will increase the thickness of the parent metal, which obviously makes for a block that is a lot stronger and less susceptible to those types of failures.”
The Messy Side Of Stroking A Small-Block
Leaving bore spacing aside, Chevy guys have been building bigger displacement small-blocks due to the amount of stroke they can package. It’s important to keep in mind that while working with the available journal diameters of a SBC crankshaft when increasing the stroke, we will be spreading the journals farther apart centerline to centerline.
Altering the bore spacing and going up a step to 4.500-inches, or even a step further to 4.600-inches would allow for more stroke and a crankshaft with more bulk in the rod throw.
“Chevy guys are building bigger cubic-inch small-blocks due to the amount of stroke they found they could get out of the factory crank journal diameter on the mains and rods,” states Kieffer. “If you are planning on maintaining the same journal spacing and diameter, but increase the stroke, we will be spreading the journal diameter farther apart, centerline to centerline. As a result, we’re making the area designed for rod throw very thin, because it’s stretched so to speak, making the weakest point the crankshaft itself.”
“Altering the bore spacing and going up a step to 4.500-inches, or even a step further to 4.600-inches would allow for more stroke and a crankshaft with more bulk in the rod throw,” Kieffer explains. “Effectively, we’re adding more material to the crankshaft (or rod throw) to make it sturdier. The first 4.600-inch bore-spaced small block that we produced was for Kenny Duttweiler of Duttweiler Performance in a V6 format, and it provided great results!”
With power adders, you will eventually reach a point of diminishing returns, where your engine exceeds the capabilities of the cylinder heads to be able to feed your powerplant with air. Your build will make more torque sooner, the problem is that it won’t be in a useable rev range – especially for drag racing, where high RPM is king.
Nitrous and forced induction are like a ventilator for your engine. It’s a way of making up for a lack of mechanical capability and increasing its ability to breath using external sources.
“Nitrous engines, like those used in Pro Mod, make up for their lack of mechanical ability to draw in a higher volume intake charge by introducing nitrous oxide into the system,” Kieffer explains.
Nitrous increases power output in two ways. Primarily, nitrous is an amazing oxidizer at higher temperatures, like those found inside an engine. When introduced into the combustion chamber it creates an oxygen rich charge far beyond what would occur naturally, allowing for more fuel and more power. Secondarily, its ability to drastically reduce the temperature of the intake charge with the push of a button increases air density, allowing you to introduce additional O2 and fuel into the engine.
“Another way to make up for mechanical limitations is with the addition of forced induction,” Kieffer continues. “With a turbo or supercharger, we’re compressing the air before it enters the combustion chamber and creating a higher oxygen content in the intake charge, again increasing fuel demands and power output.”
This is where altered bore spacing platforms also come to the rescue, by again increasing the engine displacement using a bigger bore and running cylinder heads with a greater flow rate to match, all while increasing the overall strength of the heads and block.
The advantage to using an iron block for its greater ring seal is thrown out the window with power adder builds. “Working on a nitrous build, the engine is so forgiving that it doesn’t really care about cylinder wall geometry,” says Kieffer. “That’s not to say that there aren’t some advantages to having a tighter engine, even in a boosted or power adder application. Just that the losses associated with having a poor ring seal or not as round of a bore just don’t matter as much in this scenario.”
Bolting It All TogetherWhen putting it all together, CFE’s altered bore space blocks don’t require any additional parts beyond what any other competition block would need. “A finished block will feature ductile iron sleeves that are sized in a manner that we have found to be ideal in the interest of providing the best ring seal for each individual build,” Kieffer states. “Jesel keyway lifter bushings are requested enough to be considered pretty much standard, and steel main caps seem to be the most requested, although an aluminum cap with more cross-section is an option with the big block’s larger architecture.”
Another area worth noting is cylinder head selection. When upgrading to an altered bore space block, standard spaced combustion chamber heads will no longer work and new heads will need to be purchased that match the new bore spacing. Regardless, drastically increasing the engine’s displacement and mass flow rate won’t do your power curve any favors without a way to pump that higher air volume in and out of the cylinders efficiently, so new heads should already be on your radar.
It’s important to discuss head options with CFE’s team when planning your altered bore space build to make sure that whatever head option you ultimately choose, they will be compatible with the block’s bore spacing and will be capable of handling the increase in power, cylinder pressure, heat, and airflow that comes along with your new billet block.
CFE’s billet 11-degree spread port BBC head on Chris Johnston’s ANDRA Top Alcohol Dragster.
The machining options and one piece construction of the billet cylinder heads offered by CFE will guarantee compatibility and allow your setup to perform more reliably at its highest potential – with greater fatigue resistance, a higher flow rate, and much greater repairability. Every symmetrical and spread port billet head that CFE offers is built from scratch, so your head will be custom matched to your blocks specifications, even before the CNC-machining and porting begins. Having the head custom made also gives you a much more refined list of valve train part combinations you can use to meet your goal.
CFE offers numerous port size and shape options to suit the needs of various racing applications in both billet and cast.
With the billet cores being built from scratch, the intake ports are going to be unique to the top end of the engine. For carb and fuel injection, as well as some power adder builds, custom fabricated manifolds will be required, either from CFE directly or another capable fabricator.
CFE’s billet altered bore spacing lineup is a great way to increase the reliability and service life of high output power adder engines. Its lineup covers GM’s major performance platforms in solid or water jacketed options, such as the 4.500-inch spaced LS-SBC “Fusion” block, BBCs from 4.840-inch to 5.300-inch bore spacing (with splayed or cross-bolted main caps), SBCs in 4.500-inch and 4.600-inch (600 ci), and V6 small-blocks in 4.500-inch and 4.600-inch bore spacing.
Be sure to head over to CFE Racing‘s site to check out their full range of block features and products. Details include deck height, bore sizes, cam tunnel/core sizes, lifter options, cam-to-crank dimensions, and cam height; depending on the specific platforms limitations.